The present invention relates to the cascade connection of a plurality of devices so as to enable them to communicate and function together.
In, for example, computer networks comprising a number of users there are provided communication hubs which enable the users to communicate with each other. The communication hub would typically comprise a plurality of ports to which the users would be connected, and would also comprise means to enable the ports to communicate with each other, thereby allowing the different users to communicate. As is well known the connections between the ports may be arranged in various ways, for instance the device may be what is known as a repeater, in which communications received on one port are simply output on all of the other ports, or the device may be what is known as a bridge, in which data received on any port is only output to that port for which it is destined or it may be any other form of communications device. In such an arrangement, the capacity of the network in terms of the number of users which may be connected is limited by the number of ports provided on the communications device. If it is desired to increase the capacity of the network it would of course be possible to replace the communications device with another device having an increased number of ports. However, this is generally considered not to be a good solution both in terms of wastage (the original device becomes redundant) and also in terms of product supply (a manufacturer must have in its product range a potentially large number of differently sized products).
A better approach to the problem is to provide communications devices which may themselves be connected together in what may be considered to be a xe2x80x9cstackxe2x80x9d such that when the capacity of existing equipment is exhausted further capacity may be added by simply adding an additional device to the stack. In such a stack it is necessary to provide interconnection between the stacked devices via what is known as a cascade connection in order to allow users connected to one device to communicate with users connected to other devices.
The present invention is concerned with how the cascade connection between stacked devices is implemented and, while it is useful in the above-mentioned context of computer communication networks, it is also useful in any situation where it is desired to be able to stack similar units together and connect them for intercommunication purposes.
The present invention aims to facilitate the stacking of devices by a user and to reduce the amount of manual setting up of the stack of devices which is required to be undertaken by a user.
The present invention provides a method of interconnecting a plurality of devices in a communication stack comprising providing means enabling each device to detect automatically its position within the stack.
This automatic detection provided by the present invention enables each of the devices in the stack automatically to be individually identifiable by an external management means, without the need for setting physical switches on the devices by a user, which is open to error. The automatic generation of the position identification for each device means that the stack can be re-configured by addition or removal of devices and these alterations will automatically be taken into account by the stack as a whole.
In one aspect of this invention each unit is able to detect whether it is at the top or the bottom of the stack and in the preferred implementation of this aspect it is only necessary to provide two additional connections in the cascade connection between the devices to provide this function.
In a second aspect the devices which are connected in the stack automatically generate a unique identification which can then be used for the management of the stack. In the preferred implementation this automatic ID generation requires only one additional connection in the cascade.
In a preferred development of the first aspect of the invention the devices are also provided with suitable circuitry for terminating the communication bus which is effectively provided by the cascade connection of the devices and this termination circuitry is switched on to the cascade connection if the device is at the end of the stack.
In one implementation of the invention the devices are connected in cascade by the provision of two connectors on each device and the devices are serially connected together by way of a plurality of cables between the first connector on one box and the second connector on the next box.
In such an arrangement the removal of a device in the middle of the stack causes the stack to be divided in two. In order to avoid the possible disadvantages of this, in a preferred embodiment, a further device is provided which is connected between the cables and each device which enables the removal of a communications device without breaking the cascade connection of the remaining devices.
As mentioned above this invention is particularly applicable to data communication devices in computer networks and in a particular embodiment of this invention due to the low number of extra connections which are required to perform the above-mentioned automatic functions of the cascade, it is possible to provide five independent communication links in the stack using only 25-way connectors for the cascade.